1. Field of the Invention
The present invention relates to a vehicle steering apparatus, particularly to a device for measuring torque with high accuracy for use as a vehicle steering apparatus in which resolvers are used as a turning angle detector.
2. Description of the Related Art
In general, moving or stopped vehicle wheels (in contact with a road surface) turn to a certain orientation once the steering wheel of the vehicle is turned by a driver. However, since there is a friction force acting between the wheels and the road surface, it is impossible for the wheels to turn to exactly the same angle as the steering wheel because of a transmission loss involved in a motive force transmitting process.
In order to solve the above problem, it is necessary to measure and then compensate for a difference between the turning angle of the steering wheel and that of the vehicle wheels. Traditionally, what has been in actual use is a torque sensor since it can provide such a desired function. Namely, the torque sensor can be used to measure a deviation between the turning angle of the steering wheel and that of the vehicle wheels. Meanwhile, a driving means provided independently of the torque sensor is used to rotate the vehicle wheels in response to an extent of the measured deviation In this way, it is possible to always steer the vehicle in a correct direction as directed by the driver.
FIG. 5 is a perspective view showing an external appearance of a torque sensor used in a conventional vehicle steering apparatus having the aforementioned functions. FIG. 6 is a partially enlarged explanatory view showing a part of the torque sensor shown in FIG. 5. As shown in the drawings, the conventional torque sensor comprises an input shaft 72 whose one end is combined with a steering wheel 71, an output shaft 74 whose one end is connected to the vehicle wheels, and a torsion bar 73 which is deformed to a certain extent corresponding to an actual steering operation. Further, three detection rings 66 to 68 are provided between the input shaft 72 and the output shaft 74. In fact, these detection rings 66 to 68 are all made of a magnetic material and arranged with spaces between one another at a predetermined interval between the input shaft 72 and the output shaft 74. Specifically, the first detection ring 66 is combined with the external surface of the input shaft 72 closer than any other detection rings to the steering wheel 71, and is rotatable at exactly the same angle with the steering wheel 71. The second detection ring 67 is combined with the outer peripheral surface of the central portion of the torsion bar 73. The third detection ring 68 is combined with the external surface of one end of the output shaft 74, which is the end connected to the vehicle wheels and is rotatable at substantially the same angle with the vehicle wheels.
Further, one end face of the first detection ring 66 (which is in fact an end face facing the second detection ring 67), is formed into a tooth section. Similarly, an end face of the second detection ring 67 and an end face of the third detection ring 68 (the two end faces are facing each other) are also each formed with tooth sections. Moreover, a coil (first coil) 61 is wound around an interval between the first and second detection rings 66 and 67, while another coil (second coil) 70 is wound around an interval between the second and third detection rings 67 and 68. Specifically, both of the coils 61 and 70 are all connected to a processing unit 69.
Next, the description will be given to explain an operation of the conventional torque sensor constructed in the above-described manner, which is for use as a vehicle steering apparatus in a vehicle. Namely, once a driver turns the steering wheel 71, the input shaft 72, the output shaft 74 and the torsion bar 73 are rotated. At this time, one end of the torsion bar 73 (which is connected to the steering wheel 71) is twisted larger and thus rotate more than the other end of the torsion bar 73 which is connected to the vehicle wheels. In other words, when the steering wheel 71 is turned (revolved), a friction force acting between the vehicle wheels and the road surface brings the following results. The rotation angle of the first detection ring 66 is larger than the rotation angle of the second detection ring 67, while the rotation angle of the second detection ring 67 is larger than the rotation angle of the third detection ring 68.
In this way, although there is almost no change in the mutually facing area between the tooth section of the first detection ring 66 and the second detection ring 67, there is a change in the mutually facing area between the tooth section of the second detection ring 67 and the tooth section of the third detection ring 68. For this reason, there is a change in an external magnetic flux between the second detection ring 67 and the third detection ring 68, thus causing a change in the magnetic flux passing through the second coil 70. Here, the inductances of the first and second coils 61 and 70 are set at exactly the same value. Accordingly, with the rotation of the steering wheel 71, although there is not, any change in the magnetic flux passing through the first coil 61, there is a change in the magnetic flux passing though the second coil 70. In this way, by measuring a change in an induced electromotive force of the second coil 70 with respect to an induced electromotive force of the first coil 61, it is possible to measure a rotational deviation between the steering wheel 71 and the vehicle wheels.
On the other hand, there has long been known another device called a resolver which comprises a rotary transformer as shown in FIG. 7. In fact, such a resolver includes a rotary shaft 50, a rotor 54 mounted on the rotary shaft 50, a resolver excitation winding 58 wound around the rotor, an inner core 56, and a transformer output winding 60 wound around the inner core 56. Actually, all these elements are rotatably mounted by means of bearings 51A and 51B located within a casing 52. Further, the casing 52 also encloses a stator 53, a resolver output winding 57 wound around the stator 53, an outer core 55, and a transformer excitation winding 59 wound around the outer core 55.
An excitation voltage applied to the transformer excitation winding 59 is induced in the transformer output winding 60, by virtue of an action of the rotary transformer formed by the outer core 55 and the inner core 56. The voltage induced in the transformer output winding 60 is then applied to the resolver excitation winding 58. In this way, X and Y components of the rotation angle are correspondingly outputted to the resolver output winding 57 with the rotation of the rotary shaft 50, respectively.
As described above, the conventional torque sensor shown in FIGS. 5 and 6 has three detection rings and two coils, forming a mutually facing area between the tooth section of the first and second detection rings 66 and 67, and another mutually facing area between the tooth sections of the second and third detection rings 67 and 68. In fact, there is a relative change in each of the above two mutually facing areas, and such a relative change causes a change in an induced electromotive force, so that it is possible to measure a difference between the induced electromotive forces of the first and second coils 61 and 70.
However, although the input shaft 72, the output shaft 74 and the torsion bar 73 are rotated once the steering wheel 71 is turned, at this time, one end of the torsion bar 73 connected to the steering wheel 71 is twisted larger and thus rotate more than the other end of the torsion bar 73 connected to the vehicle wheels. Accordingly, there is only a reduced change in a mutually facing area between the tooth sections of the second and third detection rings 67 and 68. As a result, it is impossible to measure torque with high accuracy. Moreover, since the torque sensor includes a large number of parts forming it, it is necessary to perform various adjustments for these parts.
On the other hand, the resolver comprising the rotary transformer shown in FIG. 7 is associated with a problem called a magnetic flux leakage. Namely, magnetic flux leaks from the outer core 55, the inner core 56, the transformer excitation winding 59 and the transformer output winding 60, which together form the rotary transformer. In fact, the magnetic flux leakage induces a sort of noise voltage in the resolver excitation winding 58 and the resolver output winding 57, thus deteriorating the measurement accuracy of the resolver. In fact, this problem is particularly remarkable with a resolver having a high sensitivity.
Accordingly, it is an object of the present invention to provide an improved device for measuring torque with high accuracy and a simple structure, thereby solving the aforementioned problems.
A device for measuring torque with high accuracy according to a first aspect of the present invention, comprises a first resolver and a second resolver which are formed in an integrated structure, each of the first and second resolvers including a rotor having a resolver excitation winding and a stator having a resolver output winding for outputting X and Y components of each rotation angle in accordance with the rotation of the rotor. In particular, one end of the rotor of the first resolver is fixed with an input shaft combined with a steering wheel, and the other end of the rotor of the second resolver is fixed with an output shaft combined with vehicle wheels, while the input shaft and the output shaft are both fixed with a torsion bar.
A device for measuring torque with high accuracy according to a second aspect of the present invention, further comprises inner cores formed on the rotors of the first and second resolvers, which the inner cores have rotary transformer output windings; outer cores formed on the stators of the first and second resolvers, which the outer cores have rotary transformer excitation windings; the inner cores, the rotary transformer output windings, the outer cores and the rotary transformer excitation windings together form a rotary transformer, providing such a structure that voltages induced in the rotary transformer output windings by virtue of the rotary transformer are applied to the resolver excitation windings of the rotors, thereby obtaining output voltages corresponding to actual rotation angles of the rotors from the resolver output windings. Specifically, a first shield plate is disposed between the inner core and the resolver excitation winding, and a second shield plate is disposed between the outer core and the resolver output winding. In particular, the first shield plate is formed with a notched hole allowing passing therethrough of a cross-over wire for use in connecting the rotary transformer output winding with the resolver excitation winding. More specifically, the notched hole is formed as having a slope and extending between the rotary transformer output winding and the resolver excitation winding.
In the device for measuring torque with high accuracy according to a third aspect of the present invention, shield plates are provided between the first resolver and the second resolver.
In the device for measuring torque with high accuracy according to a fourth aspect of the present invention, the first and second resolvers are arranged opposite to each other, the resolver excitation windings of the two resolvers are arranged opposite to each other, and the resolver output windings of the two resolvers are also arranged opposite to each other.
In the device for measuring torque with high accuracy according. to a fifth aspect of the present invention, the shield plates provided between the first resolver and the second resolve are respectively disposed between the resolver excitation windings and the resolver output windings.
In the device for measuring torque with high accuracy according to a sixth aspect of the present invention, each shield plate provided between the first resolver and the second resolver is a ring-like member.
In the device for measuring torque with high accuracy according to a seventh aspect of the present invention, the first and second shield plates are ring-like members.
In the device for measuring torque with high accuracy according to an eighth aspect of the present invention, the notched hole formed in the first shield plate is located close to either the output shaft or the input shaft.